remoteproc: Fix remaining wrong return formatting in documentation

kernel documentation specification:
"The return value, if any, should be described in a dedicated section
named Return."

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20211206191858.10741-1-arnaud.pouliquen@foss.st.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>